Don't worry about route numbers. They will just confuse you.
Turn immediately south from I-81 exit 35. Follow the major two lane White Top Road 11 miles until you cross a small bridge and come to an intersection. Look for a sign that says "White Top Road" and "Elk Garden" on the left. Turn left. Continue up the major two lane white top road another 5 miles to the trail head.
When I was there a while back, there was still no sign on the summit. That "big boulder" is very obvious, though, and there is at least one USGS survey marker on it letting you know you're at the summit. Definitely easy to overlook that you're at the top though since it's heavily forested.